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Brakeforce changed with same pedal position!

Discussion in 'Gen 2 Prius Technical Discussion' started by R-P, Apr 8, 2012.

  1. R-P

    R-P Active Member

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    I've noticed this before, and today was a very clear example: while keeping the brake in the same position, the brakingforce will sometimes suddenly let up. Today it was at 25mph (40kmh).

    I was going downhill towards an empty roundabout where I will exit at the first road. The way the road and the roundabout are build means it is simply a 180 degree sharp turn without much need for steeringadjustment, so I enter with quite high speed and was braking a little (due to the downward slope) to arrive at the desired speed, when near the bottom/entrance-to-the-roundabout, the brakes (or generator) suddenly clearly started braking less.

    Scared the shit out of me and caused me to brake quite hard.

    Anyone know this behaviour? Any solution? Some calibration-thing? I find it amazing how it can brake using the generator (to what decelerationrate will it do this?), but it's far from perfect in my car:confused:
    Not dangerous perse, but I'd call it at least very tricky.
     
  2. rcf@eventide.com

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    Were you going downhill for a while? If the traction battery reaches full (all green bars), energy is no longer diverted to the battery, and the braking force is probably decreased at that moment. I have a long downhill run when I leave the house and this sometimes happens to me, too.

    Next time, watch the battery charge indicator and try to catch it happening.

    Richard
     
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  3. KK6PD

    KK6PD _ . _ . / _ _ . _

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  4. R-P

    R-P Active Member

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    Thanks guys, I only searched in the Gen II directory or I would have found the link you posted...:D

    I think the battery was full except for one bar. Will try to see if it happens at the same spot again.
     
  5. edthefox5

    edthefox5 Senior Member

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    Nothing to do with the battery. Its the funky ABS system.

    If you hit a bump or lose traction under speed while braking it feels like you have lost total braking power for just an instant because the ABS releases braking power. That has scared the shit out of me the first few times its happened too.

    It will also throw the yellow VSC icon up on the dash for a sceond or 2 sometimes if its a big bump or traction loss.
    The ABS is very agressive.

    Its a very discussed issue here at PC.
     
  6. climateguy

    climateguy Junior Member

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    Actually, it could be either. And my bet is that it is in fact the battery, as rcf describes. Living in the mountains, our Prius demonstrates this behavior daily, and it often happens at 7 bars on the MFD. When the battery gets "full" the engine starts spinning and regenerative braking is cut. This can result in a momentary reduction in braking force. Why it sometimes happens at 7 bars instead of the full 8 bars, I don't know. My theory is that it may have something to do with battery temperature, but that's just a theory.

    I've also experienced the traction control kicking in over rough roads while braking. The feeling is similar (a momentary loss in braking force), so that is a possibility. But since the OP states the battery was at 7 bars and there's no mention of the road being particularly bumpy, I'm going with the full battery hypothesis.
     
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  7. KK6PD

    KK6PD _ . _ . / _ _ . _

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    NOT the HV battery, that has NOTHING to do with this. The +12 volt battery is what was referred to. This is tied to the ABS as stated! This is a well known problem, and you just need to depress the Brake Pedal harder!
    Listen to what Jimbo says, he's nailed it!!!
     
  8. uart

    uart Senior Member

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    I agree with climateguy, I've experienced both behaviors.

    The one that's most commonly reported here is the "funky" ABS/Regen transition thing: Hit a bump or rough/uneven surface and it transitions from regen to ABS which can sometimes result in a disconcerting loss of braking and the need for more pedal pressure.

    But I've also very clearly experienced the need for more pedal pressure when descending a hill at 8 green bars and regeneration quite suddenly switches off (due to excess HV battery charge).
     
  9. climateguy

    climateguy Junior Member

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    It always amazes me when people state with absolute conviction that they know the answer to something when they are not in a position to know the answer. Only the OP can determine whether this is being caused by the ABS over rough pavement or loss of regenerative braking due to a "full" HV battery (or, perhaps, something else entirely).

    Change in braking force with constant pedal position at 7-8 bars SOC does most definitely occur, as reported by three separate people just in this thread, and is therefore a possibility.
     
  10. jpadc

    jpadc Type before I think too often

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    Exactly what do you expect from responses on a user board? I come here because I value people sharing their views based on their experience. I tend to listen more carefully to the individuals with more experience. If you don't find that of value, then maybe user boards are not for you. Just a thought you might want to consider before criticizing the responses of others. Your choice of course.
     
  11. climateguy

    climateguy Junior Member

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    I couldn't agree more, which is why I posted what I did. Perhaps I didn't convey my thoughts clearly, though. My problem is not with KK6PD's opinion, but rather that that opinion is stated in such a way that it sounds like it's the only possibility. He says that the HV battery has "NOTHING" (all caps) to do with the OP's problem. In stating that, he limits what the OP might look for in trying to solve the problem which, ultimately, is the whole purpose of this thread.

    (BTW, if you "listen more carefully to the individuals with more experience," then you should listen to me since I've experienced both causes of what the OP reported! ;))

    Sorry if I somehow offended you, or KK6PD, whose opinions elsewhere on PC I definitely do value.
     
  12. KK6PD

    KK6PD _ . _ . / _ _ . _

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    I don't have any problem with folks disagreeing with me.
    I am not always right.
    While I have a good batting average, considering my line of work, I tend to push away items that are not usually associated with the problem at hand. Especially where others have reported repeatable results that point to some other area being the source of the problem, I then tend to accept real world reports!

    But hey if you can come up with a real world fix, or fault, that can be confirmed, I will be the first to post a "Thanks" That's why I am here! :D
     
  13. climateguy

    climateguy Junior Member

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    I can understand that. But one must be careful to not push away items simply because they are unusual to you. Yes, the usual PC-reported cause of the OP's problem is the ABS. But many of us have also experienced the HV battery cause as well. As I stated in a previous post, my wife and I experience it daily due to the terrain we drive (repeatable, real world reports), much more often than we experience the ABS cause. Since the OP stated they had 7 bars SOC at the time, it's definitely something he/she should consider (in addition to the ABS).

    At any rate, I again apologize if my previous post came off as rude. In my line of work, claiming knowledge that one doesn't definitively, provably have is perhaps the ultimate sin. Consequently, I'm very (overly?) sensitive to it. But I very much appreciate your input to PC, and hopefully it was just jpadc that took offense.

    (As for a "fix" for the OP's problem, I don't think there is one if it's the ABS or the HV battery being "full". ;) At least if it does turn out to be the HV battery, that's somewhat predictable and one can learn to expect that it's about to happen. The ABS is a far scarier feeling due to its randomness. But neither one is indicative of a mechanical problem with the brake system, so for the OP's sake, hopefully it's one or the other.)
     
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  14. uart

    uart Senior Member

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    Actually I've never had the effect occur at under 8 bars. It always occurs precisely at the point where the SOC is too high for it to accept any more regen, and as far as I can tell this is always in the 8 bar range.

    I went with this as an option however, because you can go from 7 bars to 8 bars pretty quickly and the OP may not have noticed this.
     
  15. climateguy

    climateguy Junior Member

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    Interesting. For us, it more commonly happens with 8 bars, but it's not at all unusual for it to happen at 7. I recently replaced our HV battery, and it was the same for both the original and "new" (salvage) battery, so it's apparently not specific to our battery.

    When the engine does spin-up at 7 bars and there's a momentary loss in braking force, I don't think I've ever seen it subsequently go to 8 bars. As I said in a previous post, I'm not sure why it sometimes happens at 7 bars, as it seems it should only occur at 8 bars. Maybe it's battery temperature? Maybe it's the hysteresis of the bar display? I don't know. :( And I wonder what's different about your car, driving style, terrain, or whatever that you've never seen that behavior. Huh.
     
  16. R-P

    R-P Active Member

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    Just to add, I recently just let the car roll at 50mph/80kmh and (I am 100% sure this was) at 7 bars, I felt the deceleration get lighter. Note this was 'free' deceleration, without touching any pedals.

    So this supports the theorythat (my) Prius changes *something* in the regeneration-system for an unknown (or not fully understood) reason.

    Could it be switching from MG1 to MG2 or vice versa? (Not clued in enough to even know if they can BOTH generate, but I thought I read somewhere they can???)
     
  17. qbee42

    qbee42 My other car is a boat

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    The reason isn't unknown or not fully understood. The reason is battery capacity.

    The Prius works hard to manage the battery State Of Charge (SOC). When the SOC is low, the Prius is reluctant to use electrical power, and therefor runs the ICE aggressively.

    When the SOC is high, the Prius tries to avoid adding more charge. Regenerative braking adds charge to the battery, so the Prius backs off on regeneration when the battery is nearing the high limit.

    Tom
     
  18. climateguy

    climateguy Junior Member

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    Just to add to what qbee42 said, bear in mind that there is normally regeneration occurring when just coasting. So if the battery gets to a "full" state while coasting, regeneration is stopped and the engine spins up to help slow the vehicle in place of regeneration. Thus, you don't necessarily need to be braking in order to feel a change in drag on the car when this switch-over happens.

    Nice to know I'm not the only one that this sometimes happens to at 7 bars instead of 8. Have you only had it happen at 7?
     
  19. uart

    uart Senior Member

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    I have a fairly big hill nearby, with a set of traffic lights right at the bottom. This is where I usually notice it. When I'm descending I'm riding the brakes most of the way, and the SOC usually goes up to 7 green bars. If I also get stopped by the red lights at the bottom then that last bit of extra braking often pushes it up to 8 green bars. I'm usually braking at a medium/light level, probably only about 20 to 25 MPH and usually it's pretty abrupt, the brake feel suddenly changes a little and I have to apply a little more pressure (or at least push the pedal a little further down) to continue braking at the same rate. Soon after that, usually within a few seconds of stopping at the lights, the engine starts spinning (without fuel) to dump some of that 8 bar energy.

    I'm sure that the aggressiveness of regeneration probably does change with SOC, it would make sense to do so. But I think it must be somewhat more gradual. I've never noticed anything abrupt except for the above scenario, where the regen suddenly cuts out completely at 8 green bars (just before or just as it reaches the energy dumping threshold).
     
  20. R-P

    R-P Active Member

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    I rarely see 8 bars. Note I'm in the Netherlands, who have this name cause the land is 'low'. Our highest hill is around 1000ft high and that's in another corner of the country compared to where I am. As said before, the only 'hill' I have is driving down the dyke that surrounds my city (50ft?). I don't have any heightdifferences over 60-70ft (tunnel under a waterway that goes down and then up again within half a mile).

    Maybe the Prius (as some of you agree) actively tries to avoid the 8th bar and since I don't have hills and succeeds in this 95% of the time the battery is full.

    I did not notice the engine start, but as said I didn't have 8 but 7 bars. I will try to pay extra attention to this if it happens again.